U.S. patent number 11,269,049 [Application Number 16/356,764] was granted by the patent office on 2022-03-08 for distributed aperture automotive radar system.
This patent grant is currently assigned to NXP USA, Inc.. The grantee listed for this patent is NXP USA, Inc.. Invention is credited to Arunesh Roy, Ryan H. Wu.
United States Patent |
11,269,049 |
Wu , et al. |
March 8, 2022 |
Distributed aperture automotive radar system
Abstract
A distributed radar system, apparatus, architecture, and method
is provided for coherently combining physically distributed radars
to jointly produce target scene information in a coherent fashion
without sharing a common local oscillator (LO) reference by
configuring a first (slave) radar to apply fast and slow time
processing steps to target returns generated from a second (master)
radar, to compute an estimated frequency offset and an estimated
phase offset between the first and second radars based on
information derived from the fast and slow time processing steps,
and to apply the estimated frequency offset and estimated phase
offset to generate a bi-static virtual array aperture at the first
radar that is coherent in frequency and phase with a mono-static
virtual array aperture generated at the second radar, thereby
achieving better sensitivity, finer angular resolution, and low
false detection rate.
Inventors: |
Wu; Ryan H. (San Jose, CA),
Roy; Arunesh (San Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
NXP USA, Inc. |
Austin |
TX |
US |
|
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Assignee: |
NXP USA, Inc. (Austin,
TX)
|
Family
ID: |
1000006160319 |
Appl.
No.: |
16/356,764 |
Filed: |
March 18, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20200300965 A1 |
Sep 24, 2020 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S
13/42 (20130101); G01S 13/505 (20130101); G01S
7/288 (20130101); G01S 13/878 (20130101); G01S
7/2883 (20210501); G01S 13/003 (20130101); G01S
13/931 (20130101); G01S 7/42 (20130101); G01S
7/03 (20130101); G01S 13/87 (20130101); G01S
13/584 (20130101); G01S 7/356 (20210501) |
Current International
Class: |
G01S
7/03 (20060101); G01S 13/931 (20200101); G01S
7/288 (20060101); G01S 13/00 (20060101); G01S
13/87 (20060101); G01S 13/50 (20060101); G01S
13/58 (20060101); G01S 7/35 (20060101); G01S
13/42 (20060101); G01S 7/42 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3136122 |
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Mar 2017 |
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EP |
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PCT-2018/115370 |
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Jun 2018 |
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WO |
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WO-2020158009 |
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Aug 2020 |
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WO |
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Other References
Paul Swirhun, Millimeter-Wave Circuit Design for Radar
Transceivers, Technical Report No. UCB/EECS-2013-192, Dec. 1, 2013,
https://www2.eecs.berkeley.edu/Pubs/TechRpts/2013/EECS-2013-192.pdf.
cited by applicant .
NXP, Analog, Mixed Signal and Power Management, MR2001-77 GHZ Radar
Transceiver Chipset, 2015. cited by applicant .
Jasbir Singh et al., Texas Instruments, AWR1642 mmWave sensor:
76-81-GHz radar-on-chip for short-range radar applications, 2017,
http://www.ti.com/lit/wp/spyy006/spyy006.pdf. cited by applicant
.
Florian Starzer et al., A Novel 77-GHz Radar Frontend with 19-GHz
Signal Distribution on RF-PCB Substrate, 2010 Topical Meeting on
Silicon Monolithic Integrated Circuits in RF Systems (SiRF), Jan.
11-13, 2010,
https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5422941.
cited by applicant .
Anil Kumar K V et al., Texas Instruments, Application Report,
SWRA574A--Oct. 2017, revised Dec. 2017, AWR1243
Cascade,http://www.tij.co.jp/jp/lit/an/swra574a/swra574a.pdf. cited
by applicant .
R. Feger et al., A 77-GHz FMCW MIMO Radar Based on Loosely Coupled
Stations, the 7th German Microwave Conference, Mar. 12-14, 2012,
https://ieeexplore.ieee.org/document/6185182/. cited by applicant
.
Alexander Ganis et al., A portable 3D Imaging FMCW MIMO Radar
Demonstrator with a 24.times.24 Antenna Array for Medium Range
Applications, IEEE Transactions on Geoscience and Remote Sensing,
vol. 56, Issue:1, Jan. 2018), pp. 298-312, Sep. 22, 2017. cited by
applicant .
Raza, Ahsan et al., "Thinned Coprime Array for Second-Order
Difference Co-Array Generation With Reduced Mutual Coupling", IEEE
Transactions on Signal Processing, Apr. 15, 2019, pp. 2052-2065,
vol. 67, No. 8, IEEE, Piscataway, NJ, USA. cited by applicant .
Yang, Yang et al., "Some Phase Synchronisation Algorithms for
Coherent MIMO Radar", 45th Annual Conference on Information
Sciences and Systems, Mar. 23, 2011, pp. 1-6, IEEE, Piscataway, NJ,
USA. cited by applicant .
Chun-Lin Liu, Sparse Array Signal Processing: New Array Geometries,
Parameter Estimation, and Theoretical Analysis, Thesis, Caltech,
2018. cited by applicant .
Zhe Wang et al., Nested Array Sensor With Grating Lobe Suppression
and Arbitrary Transmit-Receive Beampattern Synthesis, IEEE Access,
published Feb. 9, 2018. cited by applicant .
D. Kalogerias, et al., "Sparse sensing in colocated MIMO radar: A
matrix completion approach," IEEE International Symposium on Signal
Processing and Information Technology, 2013, pp. 000496-000502,
doi: 10.1109/ISSPIT .2013.6781930. (Year: 2013). cited by
applicant.
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Primary Examiner: Hammond, III; Thomas M
Claims
What is claimed is:
1. A distributed aperture radar system comprising first and second
small aperture radar devices that are physically distributed from
one another and connected to a radar control processing unit: the
first small aperture radar device comprising a first plurality of
transmit and receive antennas, a first local oscillator reference
clock generator, and a first signal processor component coupled to
the first plurality of transmit and receive antennas to transmit a
first radar signal and to generate a mono-static virtual array
aperture by processing target returns received at one or more
receive antennas of the first small aperture radar device in
response to the first radar signal; the second small aperture radar
device comprising a second plurality of transmit and receive
antennas, a second independent local oscillator reference clock
generator, and a second signal processor component coupled to the
second plurality of transmit and receive antennas and configured:
to process target returns and an eavesdropped signal of the first
small aperture radar device received at one or more receive
antennas of the second small aperture radar device in response to
the first radar signal with fast and slow time processing steps, to
compute an estimated frequency offset and an estimated phase offset
between the first and second small aperture radar devices based on
information derived from the fast and slow time processing steps,
and to apply the estimated frequency offset and estimated phase
offset to generate a bi-static virtual array aperture that is
coherent in frequency and phase with the mono-static virtual array
aperture; and the radar control processing unit coupled to the
first and second small aperture radar devices and configured to
produce target scene information by coherently combining the
bi-static virtual array aperture and the mono-static virtual array
aperture to construct an extended bi-static virtual array aperture
that is larger than either the bi-static virtual array aperture or
the mono-static virtual array aperture, where the first and second
small aperture radar devices each comprise a system-on-a-chip
(SOC).
2. The distributed aperture radar system of claim 1, where the
extended bi-static virtual array aperture comprises a bi-static
multiple-input multiple-output (MIMO) virtual array aperture.
3. The distributed aperture radar system of claim 1, where the
second small aperture radar device performs the fast and slow time
processing steps by applying range and doppler processing steps to
the target returns to generate a range-doppler map.
4. The distributed aperture radar system of claim 3, where the
second small aperture radar device computes the estimated frequency
offset from the range-doppler map based on doppler and range
positions of an identified eavesdropped signal peak in the
range-doppler map.
5. The distributed aperture radar system of claim 3, where the
second small aperture radar device computes the estimated phase
offset from the range-doppler map based on a detected phase of a
peak amplitude of an identified eavesdropped signal peak in the
range-doppler map.
6. The distributed aperture radar system of claim 3, where the
second signal processor component computes range fast Fourier
transforms (FFTs) and doppler FFTs on target return signals
received at the one or more receive antennas of the second small
aperture radar device to generate the range-doppler map.
7. A method for operating a distributed aperture radar system
comprising first and second small aperture radar devices that are
physically distributed from one another and connected to a radar
control processing unit, wherein the first and second small
aperture radar devices each comprise a system-on-a-chip (SOC), the
method comprising: transmitting a first radar signal from a first
transit antenna at the first small aperture radar device;
processing, at the first small aperture radar device, target
returns received at one or more receive antennas of the first small
aperture radar device in response to the first radar signal to
generate a mono-static virtual array aperture; processing, at the
second small aperture radar device, target returns and an
eavesdropped signal of the first small aperture radar device
received at one or more receive antennas of the second small
aperture radar device in response to the first radar signal with
fast and slow time processing steps; computing, at the second small
aperture radar device, an estimated frequency offset and an
estimated phase offset between the first and second small aperture
radar devices based on information derived from the fast and slow
time processing steps; applying, at the second small aperture radar
device, the estimated frequency offset and estimated phase offset
to generate a bi-static virtual array aperture that is coherent in
frequency and phase with the mono-static virtual array aperture;
and producing target scene information by coherently combining the
bi-static virtual array aperture and the mono-static virtual array
aperture to construct an extended bi-static virtual array aperture
that is larger than either the bi-static virtual array aperture or
the mono-static virtual array aperture.
8. The method of claim 7, where the first small aperture radar
device comprises a first plurality of transmit and receive
antennas, a first local oscillator reference clock generator, and a
first signal processor component coupled to the first plurality of
transmit and receive antennas to transmit the first radar signal
and to generate the mono-static virtual array aperture.
9. The method of claim 8, where the second small aperture radar
device comprises a second plurality of transmit and receive
antennas, a second independent local oscillator reference clock
generator, and a second signal processor component coupled to the
second plurality of transmit and receive antennas to generate the
bi-static virtual array aperture.
10. The method of claim 7, where the extended bi-static virtual
array aperture comprises a bi-static multiple-input multiple-output
(MIMO) virtual array aperture.
11. The method of claim 7, where computing, at the second small
aperture radar device, the estimated frequency offset and estimated
phase offset comprises by applying range and doppler processing
steps to the target returns to generate a range-doppler map.
12. The method of claim 11, where computing, at the second small
aperture radar device, the estimated frequency offset comprises
computing the estimated frequency offset from the range-doppler map
based on doppler and range positions of an identified eavesdropped
signal peak in the range-doppler map.
13. The method of claim 11, further comprising computing, at the
second small aperture radar device, range fast Fourier transforms
(FFTs) and doppler FFTs on target return signals received at the
one or more receive antennas of the second small aperture radar
device to generate the range-doppler map.
Description
BACKGROUND OF THE INVENTION
Cross-Reference to Related Applications
U.S. patent application Ser. No. 16/356,776, entitled "Distributed
Aperture Automotive Radar System With Alternating Master Radar
Devices," by inventor Ryan H. Wu, filed on even date herewith,
describes exemplary methods and systems and is incorporated by
reference in its entirety.
U.S. patent application Ser. No. 16/356,792, entitled "High
Resolution Automotive Radar System with Forward and Backward
Difference Co-Array Processing," by inventor Ryan H. Wu, filed on
even date herewith, describes exemplary methods and systems and is
incorporated by reference in its entirety.
Field of the Invention
The present invention is directed in general to radar systems and
associated methods of operation. In one aspect, the present
invention relates to an automotive radar system formed with
independent distributed radars.
Description of the Related Art
Radar systems may be used to detect the range and velocity of
nearby targets. With advances in technology, radar systems may now
be applied in many different applications, such as automotive radar
safety systems, but not every radar system is suitable for every
application. For example, 77 GHz Frequency Modulation Continuous
Wave (FMCW) Fast Chirp Modulation (FCM) radars are used as primary
sensors in Advanced Driver Assistance System (ADAS) and are used as
safety sensors in autonomous driving (AD) systems, but are not used
as the primary sensor in AD systems due to limited angular
resolution performance. To enable the use of such radar systems as
the primary sensor for driver replacement in AD systems, such
systems must provide better angular resolution, but this typically
requires larger antenna apertures, and therefore physically larger
radars. Unfortunately, the requirement of having larger radars can
conflict with other design and/or operational constraints, such as
integrating a large aperture radar into a vehicle which has
competing requirements for design, structure, and/or operation. For
example, the front of a vehicle may have design or structural
elements (e.g., headlights, design emblems, bumpers, etc.) that do
not readily admit the addition of a large aperture radar. Keeping
the size of radar sufficiently small so it can be integrated with
other parts of the vehicle implies that the aperture of the radar
is constrained and thus the angular resolution is limited.
Existing radar systems have attempted to address these challenges
by using techniques (e.g., bistatic multiple-input multiple-output
radar) which effectively combine a plurality of distributed,
smaller aperture radars to form a larger virtual aperture. However,
these techniques typically require that the distributed radars
share a common reference local oscillator (LO) signal (so the
radars operate on the same frequency and time reference) and/or
require complex and expensive modifications to hardware and
software to cross-correlate or mix target return signals with the
signals from other transmitters. Unfortunately, these requirements
may not be possible due to car integration, complexity, and/or cost
constraints which prevent such solutions from being robustly and
economically implemented. As seen from the foregoing, the existing
radar system solutions are extremely difficult at a practical level
by virtue of the challenges with achieving the performance benefits
of larger size radars within the performance, design, complexity
and cost constraints of existing radar system applications.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be understood, and its numerous objects,
features and advantages obtained, when the following detailed
description of a preferred embodiment is considered in conjunction
with the following drawings.
FIG. 1 is a simplified schematic diagram of a distributed coherent
radar system in accordance with selected embodiments of the present
disclosure.
FIGS. 2A-E are simplified diagrammatic depictions of the physical
radar apertures and MIMO virtual array apertures provided by two
distributed radars in accordance with selected embodiments of the
present disclosure.
FIG. 3 depicts simulation normalized spatial frequency or angle
spectrum of beamformer output results for three types of array
processing examples in accordance with selected embodiments of the
present disclosure.
FIG. 4 depicts an enlarged view of the simulation angle spectrum
shown in FIG. 3.
FIG. 5 illustrates a simplified flow chart showing the logic for
combining multiple distributed small-aperture radars to form a
virtually large coherent aperture in accordance with a first
selected embodiment of the present disclosure.
DETAILED DESCRIPTION
A distributed aperture radar system, hardware circuit, system,
architecture, and methodology are described for jointly producing
target scene information with multiple coherent radars that do not
require a shared common local oscillator reference. In selected
embodiments, a signal processing methodology and algorithm are
disclosed for controlling a plurality of physically distributed,
small aperture radars in a bi-static or multi-static radar
transceiver to determine the frequency offset .DELTA.f.sub.0 and
phase offset .DELTA..phi. between master and slave radars, thereby
avoiding the requirement(s) of a shared/common LO signal and/or
physically mixing or correlating the received other transmitters'
transmissions with received target returns. In other embodiments, a
distributed aperture, bi-static radar system is disclosed for
constructing an extended multiple-input-multiple-output (MIMO)
aperture by alternating the role of the master transmitting radar
among radars, thereby providing a bi-static MIMO aperture that is
larger than the combined physical size of the distributed apertures
for greatly improved angular resolution performance. In yet other
embodiments, a signal processing methodology and algorithm are
disclosed for constructing and accumulating mono-static and
bi-static MIMO virtual array outputs and then performing forward
and backward difference co-array processing and construction for
cascaded physical and virtual array processing to mitigate or
suppress spurious sidelobes in the formed radar beam pattern. By
providing hardware and software solutions for combining independent
distributed radars that are offset in both frequency and phase, the
disclosed distributed aperture radar system and methodology
efficiently provide a coherent bi-static MIMO virtual array having
an aperture that is many times larger than the total physical
apertures combined, thereby achieving better sensitivity, finer
angular resolution, and low false detection rate. And by using the
disclosed forward and backward single-frame difference co-array
beamforming signal processing techniques, sparse apertures may be
filled with virtual elements, resulting in very fine angular
resolution while suppressing false detections due to spurious
sidelobes.
In the context of the present disclosure, it will be appreciated
that radar systems may be used as sensors in a variety of different
applications, including but not limited to automotive radar sensors
for road safety systems, such as advanced driver-assistance systems
(ADAS) and autonomous driving (AD) systems. In such applications,
the radar systems are used to measure the radial distance to a
reflecting object and its relative radial velocity, and are
characterized by performance criteria, such as the angular
resolution (the minimum distance between two equal large targets at
the same range and range rate (or radial velocity) resolution cell
which a radar is able to distinguish and separate to each other),
sensitivity, false detection rate, and the like. Typically,
frequency modulated continuous wave (FMCW) modulation techniques
are used to identify the distance and/or velocity of a radar
target, such as a car or pedestrian, by transmitting FMCW modulated
signals from multiple transmit antennas so that reflected signals
from the radar target are received at multiple receive antennas and
processed to determine the radial distance and relative radial
velocity and direction for the radar target. However, with current
automotive designs, a vehicle can include multiple radars which
operate independently from one another. Typically, one radar's
transmissions are not used by any other radars, and are instead
treated as interferences which need to be avoided or suppressed to
prevent interference. Alternatively, the outputs from individual
radars are used independently or integrated in a non-coherent
fashion or fused by the tracker. Techniques for non-coherently
combining multiple front-end system-on-chip devices are known in
literature; see for example the following references: P. Swirhun,
"Millimeter-Wave Circuit Design for Radar Transceivers (2013); NXP
Fact Sheet entitled "MR2001: 77 GHZ Radar Transceiver Chipset"
(2015); and Texas Instruments publication entitled "AWR1642 mmWave
sensor: 76-81-GHz radar-on-chip for short-range radar applications"
(2017). However, non-coherent integration or track fusion does not
increase the system angular performance.
And while there are systems which combine distributed apertures to
form a larger aperture, such systems typically require that the
distributed radars share a common reference local oscillator (LO)
signal so the radars operate on the same frequency and time
reference. The common reference LO signal is usually shared via
physical wave-guide connections (e.g., PCB transmission lines,
substrate integrated waveguides, coaxial cables, etc.) or even
wireless connections that have precisely measured phase delays
within the frequency range of operation. Techniques for combining
multiple front-end system-on-chip devices with a shared,
distributed LO signal are known in literature; see for example the
following references: F. Starzer et al., "A Novel 77-GHz Radar
Frontend with 19-GHz Signal Distribution on RF-PCB Substrate," 2010
Topical Meeting on Silicon Monolithic Integrated Circuits in RF
Systems (SiRF), pp. 152-155 (2010); U.S. Patent Pub'n. No.
2016/0018511A1 to J. Nayyar et al. entitled "Distributed Radar
Signal Processing in a Radar System"; and "Texas Instruments
Application Report, AWR1243 Cascade" (October 2017, revised
December 2017). However, there are often situations where car
integration constraints prevent such connections from being
robustly and economically implemented.
As an alternative to physically sharing the LO signal, distributed
apertures can also be combined in systems, such as bi-static radar
systems, that form a single large aperture by having each radar
receive the other radars' transmissions and then cross-correlate
the target returns with the received transmissions from the other
radars for estimating target parameters. Multi-static radar
techniques are known in literature; see for example the following
references: U.S. Pat. No. 3,487,462 to D. Holberg entitled
"Bistatic radar configuration not requiring reference-data
transmission"; U.S. Pat. No. 3,812,493 to M. Afendykiw et al.
entitled "Bistatic passive radar"; and U.S. Pat. No. 4,994,809 to
K. Tung et al. entitled "Polystatic correlating radar." Such
approaches, however, require relatively complex and expensive
modifications to existing automotive radar transceiver hardware and
software because of the lack of the dedicated cross-correlator
circuitry for the reference signal.
To address these limitations from conventional solutions and others
known to those skilled in the art, reference is now made to FIG. 1
which depicts a simplified schematic diagram of a distributed
coherent radar system 100 which includes two or more distributed
radar devices 10, 20 connected to a radar controller processor 30.
In selected embodiments, each of the distributed radar devices 10,
20 may be embodied as a line-replaceable unit (LRU) or modular
component that is designed to be replaced quickly at an operating
location. Similarly, the radar controller processor 30 may be
embodied as a line-replaceable unit (LRU) or modular component.
Although two distributed radar devices 10, are shown, it will be
appreciated that additional distributed radar devices may be used.
In addition, the depicted radar system 100 may be implemented in
integrated circuit form with the distributed radar devices 10, 20
and the radar controller processor 30 formed with separate
integrated circuits (chips) or with a single chip, depending on the
application.
Each distributed radar device 10, 20 includes one or more
transmitting antenna elements TX.sub.i and receiving antenna
elements RX connected, respectively, to one or more radio-frequency
(RF) transmitter (TX) units 11, 21 and receiver (RX) units 12, 22.
For example, each radar device (e.g., 10) is shown as including
individual antenna elements (e.g., TX.sub.1,i, RX.sub.1,j)
connected, respectively, to three transmitter modules (e.g., 11)
and four receiver modules (e.g., 12), but these numbers are not
limiting and other numbers are also possible, such as four
transmitter modules 11 and six receiver modules 12, or a single
transmitter module 11 and/or a single receiver modules 12. Each
radar device 10, 20 also includes a chirp generator 112, 132 which
is configured and connected to supply a chirp input signal to the
transmitter modules 11, 21. To this end, the chirp generator 112,
132 is connected to receive a separate and independent local
oscillator (LO) signal generator 110, 130 so that the distributed
radars 10, 20 do not share a common local oscillator (LO) signal,
but are instead operated in a coordinated but non-coherent fashion
as disclosed herein. In addition, a common chirp start trigger
signal may be shared amongst the chirp generators 112, 132, though
delays are likely to be different due to the signal path
differences and programmable digital delay elements 111, 131. As a
result, the transmitter elements 11, 12 operate in a non-coherent
fashion because, even though they are programmed to transmit
identical waveforms and share a common schedule, the generated
waveforms are likely to have distinct starting frequencies, phases,
and transmitting time.
The radar system 100 also includes a radar controller processing
unit 30 that is connected to supply input control signals to the
distributed radar devices 10, 20 and to receive therefrom digital
output signals generated by the receiver modules 12, 22. In
selected embodiments, the radar controller processing unit 30 may
be embodied as a micro-controller unit (MCU) or other processing
unit that is configured and arranged for signal processing tasks
such as, but not limited to, target identification, computation of
target distance, target velocity, and target direction, and
generating control signals. The radar controller processing unit 30
may, for example, be configured to generate calibration signals,
receive data signals, receive sensor signals, generate frequency
spectrum shaping signals (such as ramp generation in the case of
FMCW radar) and/or state machine signals for RF (radio frequency)
circuit enablement sequences. In addition, the radar controller
processor 30 may be configured to program the modules (s) 11, 21 to
operate in a coordinated fashion by transmitting MIMO waveforms for
use in constructing a virtual aperture from a combination of the
distributed apertures formed by the distributed transmitting and
receiving antenna elements TX.sub.i, RX.sub.j.
In the example shown, each chirp generator 112, 132 generates a
chirp signal in response to a chirp start trigger signal generated
by the delay circuit 111, 131, and a corresponding reference local
oscillator signal LO #1, LO #2 generated by frequency synthesizer
unit 110, 130. Since the reference LO signals are independent, they
may have different frequency and phase values, which in turn
affects the frequency and phase of the generated chirp signal. The
resulting chirp signal from each generator 112, 132 is then
processed by the RF conditioning unit 113, 133 and amplified at the
power amplifier (PA) 114, 134 which amplifies the signal to a level
suitable for transmission as a radar signal by a transmitter
antenna unit TX.sub.1,i, TX.sub.2,i. Though not shown, it will be
understood that the transmitter module 11, 21 may include
additional processing circuits, such as a digital-to-analog
converter (DAC), phase shifter (or phase rotator), buffer, mixer,
filter, and the like.
The radar signal transmitted by the transmitter antenna unit
TX.sub.1,i, TX.sub.2,i may by reflected by an object, such as a
vehicle 1. Part of the reflected radar signal (e.g., mono-static
target returns) reaches receiver antenna units RX.sub.1,j at the
first distributed radar device 10, and another part (e.g.,
bi-static target returns) reaches receiver antenna units RX.sub.2,i
at the second distributed radar device 20. At each receiver module
12, 22, the received (radio frequency) antenna signal is amplified
by a low noise amplifier (LNA) 120, 140 and then fed to a mixer
121, 141 where it is mixed with the RF conditioned signal generated
by the RF conditioning unit 113, 133. The resulting intermediate
frequency signal is fed to a first high-pass filter (HPF) 122, 142.
The resulting filtered signal is fed to a first variable gain
amplifier 123, 143 which amplifies the signal before feeding it to
a first low pass filter (LPF) 124, 144. This re-filtered signal is
fed to an analog/digital converter (ADC) 125, 145 and is output by
each receiver module 12, 22 as a digital signal D1, D2, etc.
In order for each receiver module 11, 21 to be able to distinguish
transmitted radar signals from reflected radar signals, the
transmitted radar signals may be coded so they can be separated at
the receiver modules 12, 22. Such separability can be achieved with
code-division multiple access (CDMA), frequency-division multiple
access (FDMA), or time-division multiple access (TDMA) techniques.
For example, the transmitter antenna units TX.sub.i on each
distributed radar device (e.g., 10) may be controlled and
configured to transmit one at a time to form a Time-Division MIMO
aperture. In another example, each transmitter antenna unit
TX.sub.i may be controlled and configured to transmit with an
amount of frequency shift for forming Doppler-Division (DD) or
Frequency-Division (FD) MIMO apertures. In yet another example,
each transmitter antenna units TX.sub.i may be controlled and
configured to transmit all at once, but with chirps coded with
pseudo-random sequences that are orthogonal across transmitters for
forming Code-Division (CD) MIMO apertures.
Under control of the radar controller processor 30, the distributed
coherent radar system 100 is configured to operationally combine
multiple, physically separated small-aperture radars 10, 20 to
function as a single large coherent aperture radar. To this end,
the radar controller processor 30 may be configured with a
bi-static radar module 31 which is operative to combine the
distributed aperture signal results without requiring physically
mixing or correlating received target returns with the signals
received from other transmitters. However, to achieve this result,
the differences in the starting frequency and phase for the
reference LO signals must be determined before the system can
function as a single radar by coordinating the distributed radar
devices 10, 20 to operate in a coherent fashion. Accordingly, the
bi-static radar module 31 includes a frequency/phase measurement
module 37 that produces frequency offset measurements
(.DELTA.f.sub.0) and phase offset measurements (.DELTA..phi.). By
using the frequency/phase measurement module 37 to compute or
measure the frequency and phase offsets between the distributed
radars 10, 20, one may compensate for the differences and the
different radar signals may then be processed in a coherent fashion
as if it is a single radar. Note that the time offsets
(.DELTA.t.sub.0) amongst distributed radars are assumed to be known
which is measured at the time of factory installation and system
integration and testing.
In addition to the bi-static radar aperture construction, the radar
controller processor 30 may be configured to construct and
accumulate multiple-input-multiple-output (MIMO) array outputs to
form a MIMO aperture. To this end, the radar controller processor
may be configured with a MIMO virtual array module 38 which is
operative to alternate the transmitting "master" role among the
distributed radar devices 10, 20 so that an extended MIMO aperture
can be formed based on MIMO radar principles. In operation, the
MIMO virtual array module 38 sequentially selects each of the
distributed radar devices 10, 20 to serve as the "master" radar
while the remaining radar devices operate as "slave" radar(s) until
all of the distributed radar devices 10, 20 have been selected as
the master unit once. The selected master-unit radar device
transmits radar waveforms and the slave-unit radar device(s)
directionally receive and process the master radar's transmitted
waveforms using identical range and Doppler processing steps for
normal radar waveforms. Applying the estimated frequency and phase
offsets (.DELTA.f.sub.0, .DELTA..phi.) computed by the
frequency/phase measurement module 37, each slave radar produces
coherent target measurements which the radar controller processor
30 uses to construct and accumulate mono-static and bi-static MIMO
array outputs. The resulting bi-static MIMO aperture is even larger
than the combined physical size of the distributed apertures, which
results in greatly improved angular resolution performance. If
there are multiple sections of the resulting MIMO array, the MIMO
virtual array module 38 may be configured to identify and select
the least-sparse section of the MIMO array to compute a first set
of beamforming outputs.
As will be appreciated, the spacing and arrangement of the
transmitting and receiving antenna elements TX.sub.i, RX.sub.j may
result in the construction of a sparse bi-static MIMO array (e.g.,
contains holes or gaps), resulting in high grating lobes in the
formed radar beam pattern. To address the potential grating or
spurious lobe issues of sparse arrays, the radar controller
processor 30 may be configured with a co-array processing module 39
which is operative to perform forward and backward difference
co-array processing and cascaded physical and visual array
processing as a mitigation technique for suppressing the spurious
sidelobes. In operation, the radar controller processor 30 uses the
co-array processing module 39 to construct forward and backward
difference co-array outputs based on the MIMO array outputs. If the
formed difference co-array is uniformly spaced, the radar
controller processor 30 may perform spatial smoothing on the
difference co-array outputs. In addition or in the alternative, the
radar controller processor 30 may be configured to compute a second
beamforming output based on the (spatially-smoothed) co-array
outputs, and then compute a composite beamforming output based on
the first and the second beamforming outputs. Based on this
processing, the radar controller processor 30 may then generate and
output the target Range-Doppler-Angle map data.
Frequency/Phase Measurement
As indicated above, selected embodiments of the present disclosure
provide a method and apparatus for determining or measuring
frequency and phase differences between the distributed radar
devices 10, 20 for use in constructing a large, coherent virtual
aperture radar from multiple physically separated or distributed
small-aperture radars by compensating the distributed radars' radar
measurements for processing in a coherent fashion. In the depicted
distributed coherent radar system 100, this is illustrated with the
example of a radar controller processor 30 which coordinates the
coherent operation of two distributed radar devices 10, 20, each
consisting of three transmit channels, four receiving channels, and
an independent reference LO generator.
For illustration purposes, the first radar device 10 is selected as
the master unit to implement a time-division MIMO process wherein a
first transmit antenna (TX.sub.1,1) is radiating while the rest of
the transmit antennas (TX.sub.1,2, TX.sub.1,3) are not radiating.
By design, the second radar device 20 eavesdrops on the transmit
signal from the first transmit antenna TX.sub.1,1 to receive the
transmit signal and perform fast-time and slow-time processing on
the received signal in the same way the second radar device 20
processes its own target returns. In FIG. 1, this is shown with the
second radar device which receives the eavesdropping propagation
channel(s) at one or more of the receive antennas (RX.sub.2,j).
After processing by the receiver modules 12, 22, the digital
signals D1, D2 are each processed by the fast-time (range) FFT
module 32 and slow-time (Doppler) FFT module 33, thereby generating
the range-Doppler map (RDM). Because the eavesdropped signal is
strong, it is easily identifiable from target returns in the
range-Doppler map.
By premeasuring the propagation delays of the eavesdropping
channels (e.g., at factory installation or during system
integration and testing), the range position of the (strongest)
eavesdropping peak can be found easily. As described more fully
hereinbelow, the frequency/phase measurement module 37 may be
configured to use the Doppler and peak amplitude phase information
of the (strongest) eavesdropping peak to derive the frequency
offset .DELTA.f.sub.0 and phase offset .DELTA..phi. between the
transmitting radar 10 and the eavesdropping radar 20, respectively.
If eavesdropping signals from multiple receivers are available,
they can be coherently combined by compensating for the
pre-measured phase differences between the receiving antennas of
the eavesdropping propagation channels and then vectorially summed.
This is equivalent to forming a directional receiving beam in the
direction of the transmitting antenna (e.g., TX.sub.1,1).
Alternatively, a receiver channel at the slave radar device 20 may
be dedicated for eavesdropping on the transmission of the master
radar device 10 by connecting the receiver channel to a directional
antenna pointing at the master radar device 10. This may be needed
if the propagation environment dictates higher gain.
While any suitable sequence of processing steps may be used by the
frequency/phase measurement module 37 to measure or compute the
frequency offset .DELTA.f.sub.0 and phase offset .DELTA..phi., an
example derivation sequence is illustrated with reference to the
distributed coherent radar system 100 wherein the transmitter
module (e.g., 11) in the first radar device 10 is selected as the
master radar to transmit with the transmit antenna TX.sub.1,1, and
where the eavesdropping radar receivers (e.g., RX.sub.2,1,
RX.sub.2,2, RX.sub.2,3, RX.sub.2,4) in the second radar device 20
operate as slave radars. In this arrangement, the frequency offset
(f.sub.1-f.sub.2) between the slave and master radar may be denoted
as .DELTA.f.sub.0 (in Hz), the time offset (t.sub.1-t.sub.2) may be
denoted as .DELTA.t.sub.0 (in seconds), and the phase offset
(.phi..sub.1-.phi..sub.2) between the radars may be denoted as
.DELTA..phi. (in radians). With reference to the chirp generators
112, 132, the starting frequency of the linear-frequency modulation
(LFM) chirp may be denoted as f.sub.0 (in Hz), and the chirping
rate may be denoted as f (in Hz/sec). Without loss of generality,
it may be assumed that the initial phase and transmitting time are
both zeros, and the amplitude is "one."
With this understanding, the received radar signal y.sub.ms(t) at
the output of the mixer 141 of the slave radar's receiver 21 can be
modeled as shown below:
.times.'.times..times..times..times..times..times..times..times..times..t-
imes..times..times..function..times..times..times..pi..function..times..ti-
mes..times..times. ##EQU00001##
'.times..times..times..times..times..times..times..times..function..times-
..times..DELTA..times..times..phi..times..times..times..times..pi..functio-
n..DELTA..times..times..times..DELTA..times..times..times..DELTA..times..t-
imes..times..DELTA..times..times. ##EQU00001.2##
.times.'.times..times..times..times..times..times. ##EQU00001.3##
.times..function..times..times..times..function.
.function..times..times..times..DELTA..times..times..phi..times..times..t-
imes..times..pi..DELTA..times..times..times..DELTA..times..times..times..D-
ELTA..times..times..times..DELTA..times..times..times..times..times..times-
..times..times..times..times..pi..function..DELTA..times..times..DELTA..ti-
mes..times..times..DELTA..times..times..times..times..DELTA..times..times.-
.times..times..times..times..times..pi..function..DELTA..times..times..tim-
es..DELTA..times..times..times..DELTA..times..times..times..DELTA..times..-
times..DELTA..phi..times..pi. ##EQU00001.4##
The instantaneous frequency f.sub.ms(t) at the mixer's output can
be obtained by taking the time derivative of the received signal at
the mixer output y.sub.ms(t), as shown below.
Instantaneous Frequency at Mixer Output:
.times..times..times..times..function..times..times..times..pi..times..ti-
mes..DELTA..times..times..DELTA..times..times..DELTA..times..times..times.-
.times..times..DELTA..times..times..times..DELTA..times..times..times..DEL-
TA..times..times..times..times..DELTA..times..times..times..DELTA..times..-
times..DELTA..times..times..phi..times..pi..times..DELTA..times..times..DE-
LTA..times..times..times..DELTA..times..times..DELTA..times..times..times.
##EQU00002##
As shown by the equation model f.sub.ms(t), the instantaneous
frequency for f.sub.ms(t) varies linearly with time at a rate of
.DELTA.{dot over (f)}. Knowing that the instantaneous frequency
f.sub.ms(t) corresponds to the time delay of the signal and knowing
that the rate of the instantaneous frequency f.sub.ms(t)
corresponds to the Doppler shift, it is seen that the value
".DELTA.f" is an observable quantity that can be directly derived
from the Doppler position of the identified eavesdropped signal
peak on the range-Doppler map (RDM) generated at the output of the
slow-time (Doppler) FFT module 33. In FIG. 1, this is illustrated
with the frequency/phase measurement module 37 receiving an input
from the slow-time (Doppler) FFT module 33 which generates the
range-Doppler map. To obtain a higher accuracy estimate, the
slow-time (Doppler) FFT module 33 may perform oversampling (e.g.,
zero-padding the slow-time samples for FFT) of the received radar
signal.
Based on the first observation--that the value ".DELTA.{dot over
(f)}" can be directly derived from the Doppler position of the
identified eavesdropped signal peak on the range-Doppler map
(RDM)--it is seen that the equation for the instantaneous frequency
model for f.sub.ms(t)=(.DELTA.f.sub.0+({dot over (f)}+.DELTA.{dot
over (f)}).DELTA.t.sub.0)+.DELTA.{dot over (f)}t is an observable
quantity that can be estimated from fast-time FFT signal generated
at the output of the fast-time (Range) FFT module 32. In
particular, it corresponds to the range position of the
eavesdropped signal's peak on the range-Doppler map (RDM) generated
by the slow-time (Doppler) FFT module 33. In FIG. 1, this is
illustrated with the frequency/phase measurement module 37
receiving an input from the slow-time (Doppler) FFT module 33 which
generates the range-Doppler map. As will be appreciated, a higher
accuracy estimate can be obtained by performing oversampling (e.g.,
zero-padding) at the fast-time (Range) FFT module 32. Since
.DELTA.t.sub.0 is a pre-measured known value, .DELTA.{dot over (f)}
is obtained from the Doppler position of the identified
eavesdropped signal peak, and {dot over (f)} is the known chirp
rate, it can be seen that the value .DELTA.f.sub.0 can be solved
from the equation for the instantaneous frequency model for
f.sub.mx(t).
Based on the derived equation for the mixer output model
y.sub.mx(t), it is seen that the instantaneous frequency equation
term
.DELTA..times..times..times..DELTA..times..times..times..DELTA..times..ti-
mes..times..DELTA..times..times..DELTA..phi..times..pi.
##EQU00003## is an observable quantity which corresponds to the
phase of the peak amplitude of the eavesdropped signal, and can be
estimated by dividing the fast-time FFT's phase with 2.pi.. Given
known and obtained values of f.sub.0, .DELTA.f.sub.0,
.DELTA.t.sub.0, and {dot over (f)}, and assuming further that
.DELTA.{dot over (f)} is negligible (in fact, the entire 1/2({dot
over (f)}+.DELTA.{dot over (f)}).DELTA.t.sub.0.sup.2 is likely
negligible), .DELTA..phi. can therefore be solved.
To measure or compute the frequency offset measurements
(.DELTA.f.sub.0) and phase offset measurements (.DELTA..phi.)
between the distributed radars, the frequency/phase measurement
module 37 is connected to receive the results of the processing
steps 32-36 in the bi-static radar module 31, either in the form of
the target tracks (TRACKS) generated by the target tracking module
36 or directly from the intermediate processing stages, such as the
fast-time (range) FFT module 32 and/or the slow-time (Doppler) FFT
module 33. With these inputs, the frequency/phase measurement
module 37 is also configured with control code and data structures
to represent the signal models for the mixer output signal
y.sub.mx(t) and instantaneous mixer output frequency f.sub.ms(t)
and to produce the frequency offset measurements (.DELTA.f.sub.0)
and phase offset measurements (.DELTA..phi.) between the
distributed radars. Applying the derived frequency and phase offset
measurements as compensations to the slave measurements (or,
alternatively, to the master measurements), the radar controller
processor 30 effectively compensates for differences and the
radars' signals so that they may be processed in a coherent fashion
as if the two radars are a single coherent radar.
Bi-Static MIMO Virtual Array Aperture with Alternating Master
Radars
As indicated above, selected embodiments of the present disclosure
provide a method and apparatus for constructing a bi-static
multiple-input-multiple-output (MIMO) virtual array aperture from
multiple, distributed smaller apertures by alternating the role of
the master transmitting radar among radars, thereby providing
virtual array aperture that is larger than the combined physical
size of the distributed, smaller apertures for greatly improved
angular resolution performance. In the depicted distributed
coherent radar system 100, this is illustrated with the example of
a radar controller processor 30 where the MIMO virtual array module
38 coordinates the coherent operation of two distributed radar
devices 10, 20 by alternating the role of the master radar between
them.
As an initial step, the first radar device 10 is selected as the
master unit which implements a time-division MIMO process wherein a
first transmit antenna (TX.sub.1,1) is selected to transmit or
radiate radar signals while the rest of the transmit antennas
(TX.sub.1,2, TX.sub.1,3) are not radiating. Receiver antennas
RX.sub.1,1 to RX.sub.1,4 of the selected master radar device and
receiver antennas RX.sub.2,1 to RX.sub.2,4 of the slave radar
device 20 receive and process the mono-static and bi-static target
returns, as shown. Subsequently, the second transmit antenna
(TX.sub.1,2) and third transmit antenna (TX.sub.1,3) of the
selected master radar device 10 are sequentially selected to
transmit or radiate radar signals that are received and processed
as target returns at the receiver antennas RX.sub.1,1-RX.sub.1,4
and RX.sub.2,1-RX.sub.2,4. Based on the target returns from the
master radar device 10 received at the receiver antennas
RX.sub.1,1-RX.sub.1,4 of the slave radar device 20, a first
mono-static MIMO virtual array aperture may be formed. In addition,
a second bi-static MIMO virtual array may be formed from the target
returns from the master radar device 10 received at the receiver
antennas RX.sub.2,1-RX.sub.2,4 of the slave radar device 20.
Together, the mono-static MIMO aperture and bi-static MIMO aperture
form a bi-static MIMO virtual aperture consisting of antenna
element positions being the vectoral sum of the transmit antenna
element position and the receiver antenna element position.
To provide additional details for an improved understanding of
selected embodiments of the present disclosure, reference is now
made to FIGS. 2A-C which provide simplified diagrammatic depictions
of the physical radar apertures and resulting MIMO virtual array
apertures which may be generated by distributed radar devices 201,
202 in accordance with selected embodiments of the present
disclosure. In particular, FIG. 2A illustrates a distributed
arrangement 200A of first and second radar devices 201, 202, each
having three transmit antennas and three receiver antennas which
are symmetrically positioned and distributed in relation to one
another. In particular, the first distributed radar device 201
includes three physical transmit antennas T.sub.1,1-T.sub.1,3, with
three physical receiver antennas R.sub.1,1-R.sub.1,3 positioned
between the first and second physical transmit antennas T.sub.1,1,
T.sub.1,2. In the depicted example of a mirrored arrangement, the
second distributed radar device 202 includes three physical
transmit antennas T.sub.2,1-T.sub.2,3, with three physical receiver
antennas R.sub.2,1-R.sub.2,3 positioned between the second and
third physical transmit antennas T.sub.2,2, T.sub.2,3. Without loss
of generality, the physical antennas are shown as being positioned
in a linear fashion, but may be arranged in non-linear fashion.
To illustrate an example virtual array aperture that may be formed
with the distributed radar devices 201,202 when the first radar
device 201 is selected as the master radar, reference is now made
to FIG. 2B which illustrates a first MIMO virtual array aperture
200B formed by transmitting radar signals from the three
transmitting antennas T.sub.1,1-T.sub.1,3 of the first radar device
201 which are received at the receiving antennas
R.sub.1,1-R.sub.1,3, R.sub.2,1-R.sub.2,3 from both radar devices
201, 202. On the left side, the mono-static MIMO virtual array
elements 204 are generated by the receiving antennas
R.sub.1,1-R.sub.1,3 on the master radar device 201 which receive
radar transmit signals which are sequentially radiated by the three
transmitting antennas T.sub.1,1-T.sub.1,3 of the master radar
device 201. On the right side, the bi-static MIMO virtual array
elements 205 are generated by the receiving antennas
R.sub.2,1-R.sub.2,3 on the slave radar device 202 which receive
radar transmit signals which are sequentially radiated by the three
transmitting antennas T.sub.1,1-T.sub.1,3 of the master radar
device 201. As a result, the first MIMO virtual array aperture 200B
has more elements than the physical array 200A and occupies a
larger (wider) area. Since angular resolution is inversely
proportional to aperture size, the MIMO virtual array aperture 200B
provides improved angular resolution (as compared to the physical
array 200A). However, it is also seen that the first MIMO virtual
array aperture 200B is a "sparse" array which contains holes or
gaps between the virtual array elements.
In accordance with selected embodiments of the present disclosure,
the size of the MIMO virtual array aperture may be increased
further by also selecting the second radar device 202 to operate as
the master unit so that the first radar device 201 operates as the
slave unit. In this arrangement where the second radar device 202
is selected as the master unit and the first radar device 201 is
selected as the slave unit, the three transmitting antennas
T.sub.2,1-T.sub.2,3 of the second (master) radar device 202 are
sequentially used to generate target returns at the receiving
antennas R.sub.1,1-R.sub.1,3, R.sub.2,1-R.sub.2,3 from both radar
devices 201, 202. Where there is no shared or common reference LO
signal for the radar devices 201, 202, the ability to switch the
"master unit" role between radar devices 201,202 requires that the
frequency and phase offset measurement values be obtained and
applied to compensate the slave (or reversely to the master's)
measurements and thereby enable coherent processing of the combined
target returns from the radar devices 201, 202.
After using both radar devices 201, 202 as "master" units to
transmit radar signals from all of the transmit antennas
T.sub.1,1-T.sub.1,3, T.sub.2,1-T.sub.2,3, a second MIMO virtual
array aperture 200C may be formed, as shown in FIG. 2C. As
depicted, the second MIMO virtual array aperture 200C includes a
first set of mono-static MIMO virtual array elements 206 generated
by the receiving antennas R.sub.1,1-R.sub.1,3 when receiving
sequentially transmitted radar transmit signals from the
transmitting antennas T.sub.1,1-T.sub.1,3 of the
(master-designated) first radar device 201. In addition, the second
MIMO virtual array aperture 200C includes a second set of
mono-static MIMO virtual array elements 208 generated by the
receiving antennas R.sub.2,1-R.sub.2,3 when receiving sequentially
transmitted radar transmit signals from the transmitting antennas
T.sub.2,1-T.sub.2,3 of the (master-designated) second radar device
202. Finally, the second MIMO virtual array aperture 200C includes
a third set of bi-static MIMO virtual array elements 207 generated
by the receiving antennas R.sub.1,1-R.sub.1,3, R.sub.2,1-R.sub.2,3
when receiving sequentially transmitted radar transmit signals from
the transmitting antennas T.sub.1,1-T.sub.1,3, T.sub.2,1-T.sub.2,3
of the radar device 201. As indicated with the grouping box 209,
the third set of bi-static MIMO virtual array elements 207 includes
redundant or overlapping contributions from the first and second
radar devices when acting in their respective master unit roles. By
virtue of alternating the master role amongst the distributed radar
devices 201, 202, the second MIMO virtual array aperture 200C has
more elements than the virtual array 200B and occupies a larger
(wider) area since it is based on all six transmit antennas
T.sub.1,1-T.sub.1,3, T.sub.2,1-T.sub.2,3 and receiver antennas
R.sub.1,1-R.sub.1,3, R.sub.2,1-R.sub.2,3. As a result, the larger
second MIMO virtual array aperture 200C provides improved angular
resolution as compared to the first MIMO virtual array aperture
200B. However, it is also seen that the second MIMO virtual array
aperture 200C is still a "sparse" array which contains holes or
gaps between the virtual array elements, though the third set of
bi-static MIMO virtual array elements 207 is the least-sparse
contiguous array section.
While the second MIMO virtual array aperture 200C is described with
reference to alternating the master radar role between two
distributed radars 201,202, it will be appreciated that the
principle can be readily extended to additional radars. In
addition, the benefits of alternating master radar transmissions
can be extended to non-LFM (Linear Frequency Modulated) chirp
radar, as well as to other forms of MIMO besides TD-MIMO.
Forward and Backward Difference Co-Array Processing
As indicated above, the distributed nature of the physical
apertures to be combined (e.g., 201, 202) result in the formation
of larger MIMO virtual array apertures (e.g., 200C) which are
likely sparse (i.e., contains holes or gaps) and not entirely
filled by virtual antenna elements by virtue of the Nyquist
sampling requirements, and the resulting formed beams contain
spurious side lobes because of under sampling and/or non-uniform
sampling in the spatial domain. The presence of grating lobes, or
spurious side lobes in general, increase the likelihood of false
target detections in the angular domain.
To address these limitations and others known to those skilled in
the art, selected embodiments of the present disclosure provide a
signal processing apparatus, methodology and algorithm for
constructing and accumulating mono-static, bi-static, and
multi-static MIMO virtual array outputs and then performing forward
and backward difference co-array processing and construction for
cascaded physical and virtual array processing to mitigate or
suppress spurious sidelobes in the formed radar beam pattern. In
the depicted distributed coherent radar system 100, this is
illustrated with the example of a radar controller processor 30
where the co-array processing module 39 constructs mono-static and
bi-static MIMO virtual apertures, and then performs forward and
backward difference co-array construction to mitigate the potential
grating lobe or spurious lobe issue of sparse arrays. The generated
co-array output may be further weighted with a windowing function
to suppress the sidelobes in the formed beam pattern. In addition,
the generated array beam pattern may be further weighted with the
beam pattern formed by a contiguous or less-sparse section of the
MIMO virtual aperture to further suppress the spurious lobes.
While any suitable sequence of processing steps may be used by the
co-array processing module 39 to perform forward and backward
difference co-array processing to mitigate the spurious sidelobes
due to spatial under and non-uniform sampling, an example
processing sequence is illustrated with reference to the MIMO
virtual array aperture 200C which is further processed to generate
the MIMO virtual army aperture 200D shown in FIG. 2D. In particular
and as described more fully hereinbelow, the forward and backward
difference co-array is first constructed and then the receive beam
is formed based on the outputs of the constructed virtual array.
For each range-Doppler cell to be processed, its antenna outputs
across the formed MIMO array are further processed according to the
following steps. Without loss of generality, linear array with
equally spaced spatial samples are assumed.
With reference to the MIMO virtual array aperture 200C shown in
FIG. 2C, the i-th MIMO virtual array antenna element's position may
be denoted as x.sub.i=n.sub.i*d, where d is the unit element
spacing in meters and n.sub.i is an integer. Ideally, d should be
half wavelength for sampling the entire 180-degree field of view
without ambiguity. In practice, the antenna's field of view is
smaller than 180 degrees, so a larger spacing than half wavelength
may be used.
The forward and backward difference co-array construction starts by
constructing element pairs with respect to difference element-pair
spacing. In an example where there are four antenna elements where
[x.sub.1, x.sub.2, x.sub.3, x.sub.4]=[1, 2, 3, 5]*d, then the
co-array virtual element is denoted x.sub.i,j=x.sub.i-x.sub.j.
For construction of the forward difference co-array, all
combinations resulting in zero or positive difference spacing are
listed as the example below: x.sub.1,1=0 x.sub.2,1=d x.sub.3,1=2d
x.sub.4,1=4d x.sub.2,2=0 x.sub.3,2=d x.sub.4,2=3d x.sub.3,3=0
x.sub.4,3=2d x.sub.4,4=0
Upon grouping the antenna pair indices by non-negative difference
spacing values, the following list is constructed:
TABLE-US-00001 Difference Co-Array Element Spacing Antenna Pair
Indices 0 x.sub.1, 1, x.sub.2, 2, x.sub.3, 3, x.sub.4, 4 d x.sub.2,
1, x.sub.3, 2 2d x.sub.3, 1, x.sub.4, 3 3d x.sub.4, 2 4d x.sub.4,
1
The pair-wise difference operation indicates that a difference
co-array aperture of the size of five (5) elements can be
constructed. The formed element outputs shall be calculated as
follows.
First, the i-th MIMO antenna output is denoted as y.sub.i, which is
the k-th difference co-array element's output. Based on antenna
pair indices {x.sub.i1,j1, . . . , x.sub.iM,jM}, the k-th
difference co-array element should be calculated as
.times..times..times. ##EQU00004## The resulting forward difference
co-array element output is provided below:
TABLE-US-00002 Difference Co- Antenna Array Element Pair Spacing
Indices Virtual element output 0 x.sub.1,1, x.sub.2,2, x.sub.3,3,
x.sub.4,4 .times..times..times..times..times. ##EQU00005## D
x.sub.2,1, x.sub.3,2 .times..times..times. ##EQU00006## 2d
x.sub.3,1, x.sub.4,3 .times..times..times. ##EQU00007## 3d
x.sub.4,2 z.sub.3 = y.sub.4y.sub.2.sup.* 4d x.sub.4,1 z.sub.4 =
y.sub.4y.sub.1.sup.*
As seen from above, the outputs {z.sub.0, z.sub.1, . . . z.sub.4}
are then used as the outputs corresponding to an antenna array with
element positions {0, d, 2d, 3d, 4d}. Angle processing, such as
beamforming, can then be carried out.
In similar fashion, the backward difference co-array may be
constructed by denoting x.sub.i,j=x.sub.i-x.sub.j for forward
difference co-array, and then selecting all combinations resulting
in non-positive difference spacing to calculate and the outputs
based on the same principle. Continuing with the previous example,
the pair indices and outputs of the elements of the backward
difference co-array are identified as the following backward
difference co-array element output table:
TABLE-US-00003 Difference Co- Antenna Array Element Pair Spacing
Indices Virtual element output -4d x.sub.1,4 z.sub.-4 =
y.sub.1y.sub.4.sup.* = z.sub.4.sup.* -3d x.sub.2,4 z.sub.-3 =
y.sub.2y.sub.4.sup.* = z.sub.3.sup.* -2d x.sub.1,3, x.sub.3,4
.times..times..times. ##EQU00008## -d x.sub.1,2, x.sub.2,3
.times..times..times. ##EQU00009## 0 x.sub.1,1, x.sub.2,2,
x.sub.3,3, x.sub.4,4 .times..times..times..times..times.
##EQU00010##
As seen from above, the virtual element output can be derived from
the forward difference co-array outputs by taking the complex
conjugate.
The final aperture is constructed based on the combined forward and
backward difference co-arrays. In this example, a virtual array of
nine (9) elements is formed with antenna positions {-4d, -3d, -2d,
-d, 0, d, 2d, 3d, 4d}. Note that, if FFT is used for estimating
target angles and there are any uniform linear array positions
missing an output, zero filling should be performed to provide
outputs for the missing array positions. Also note that, while
conventional approaches for designing difference co-arrays (e.g.,
the minimum redundancy array (MRA) technique) seek to minimize the
redundancy by maximizing the forward difference co-array without
any holes, selected embodiments of the present disclosure seek to
maintain some redundancy since the averaging effect between the
overlapping contributions is helpful for reducing spurious
sidelobes. In this way, a balance may be achieved between the
design objectives of creating a large aperture and an evenly spread
redundancy. In selected embodiments, every virtual antenna array
element results from an equal number of averaging
contributions.
To provide additional details for an improved understanding of
selected embodiments of the present disclosure, reference is now
made to FIGS. 2D-E which provide simplified diagrammatic depictions
of different MIMO virtual array apertures 200D, 200E that may be
generated by two distributed radar devices 201, 202 in accordance
with selected embodiments of the present disclosure. In particular,
FIG. 2D illustrates a third MIMO virtual array aperture 200D in
which a forward difference co-array is formed on top of the third
MIMO virtual array aperture 200C formed with mono-static and
bi-static MIMO virtual arrays 206-208. With the third MIMO virtual
array aperture 200D, the size of the aperture remains unchanged,
but the aperture is fuller (e.g., fewer holes), resulting in lower
spurious sidelobes. In FIG. 2E, a fourth MIMO virtual array
aperture 200E in which a forward and backward difference co-array
is formed on top of the mono-static and bi-static MIMO virtual
arrays 206-208. With the fourth MIMO virtual array aperture 200E,
the size of the aperture is almost doubled (as compared to the
third MIMO virtual array aperture 200D) and the aperture is fuller,
resulting in improved angular resolution and improved spurious side
lobe performance.
While the difference co-array processing techniques disclosed
hereinabove improve the angular resolution and reduce the spurious
side lobes, there may be additional need for suppressing the
spurious side lobes. To this end, the co-array processing module 39
may be configured to further reduce the spurious side lobes by
spatially smoothing the forward/backward difference co-array
element outputs in the forward direction. As will be appreciated,
spatial smoothing is a technique used in array signal covariance
matrix construction for the purpose of increasing the matrix rank
as well as decorrelating coherent signals. As disclosed herein,
spatial smoothing may be used for improving the arrival signal's
progressive phase change measurement by averaging out error
contributions. The co-array processing module 39 may be operatively
configured to define a size of the spatially smoothed aperture
size. If the smoothed aperture is of the same size as the original
aperture size, no spatial smoothing is performed. However, if the
smoothed aperture size is smaller, then a sliding-window averaging
operation is taken to produce the averaged outputs. Note that the
spatial smoothing requires a virtual array of equally spaced
antenna elements. If the virtual array does not have equally spaced
antenna elements, this method does not apply.
In addition or in the alternative, the co-array processing module
39 may be configured to further reduce the spurious side lobes by
producing a composite beam forming output. To this end, the
co-array processing module 39 may be configured to multiply the
beam forming output of the forward/backward difference co-array
(with or without spatial smoothing applied) with the beam forming
output of a section of the MIMO virtual array. The selected MIMO
virtual array section ideally should not be under-sampled (e.g., a
section that forms a uniform linear array). In lieu of a filled
section of array, a section that is least sparse (i.e., a few holes
allowed) should be selected (e.g., section 207 indicated in FIG.
2C). By doing so, the spurious sidelobes in the output of the
difference co-array process are greatly suppressed.
To illustrate the improved side lobe suppression benefits of the
different array processing techniques disclosed herein, reference
is now made to FIGS. 3-4 which depict a first view (FIG. 3) and
enlarged view (FIG. 4) of simulation of normalized spatial
frequency or angle spectrum (hereinafter, angle spectrum) of
beamformer output results for three types of array processing
examples for resolving three closely-spaced targets in accordance
with selected embodiments of the present disclosure. In the
simulation, the locations of the three targets is shown,
respectively, with the true target angle or spatial frequencies
(corresponding to target directions) 301-303. In addition, the
beamforming angle spectrum 304 is generated using Discrete Fourier
Transform (DFT) of a single radar MIMO array, such as the array
output of the MIMO virtual array of a single radar aperture. An
example single radar array would be the mono-static MIMO virtual
array 204 of FIG. 2B.
With the angle spectrum 305, there is shown the beamforming output
generated using DFT of the dual radar MIMO array output. In an
example embodiment, the angle spectrum 305 is generated using DFT
of the MIMO virtual array of two distributed radars that are
combined by alternating the master role between radar devices, such
as the bi-static MIMO virtual array 200C of FIG. 2C.
With the angle spectrum 306, there is shown the beamforming output
generated using Fast Fourier Transform (FFT) of the dual radar MIMO
array output with a forward-backward difference co-array. In an
example embodiment, the angle spectrum 306 is generated using FFT
of the virtual output of a MIMO Forward/Backward Difference
Co-array of two distributed radars, such as the bi-static MIMO
virtual array 200E of FIG. 2E.
The simulation angle spectrums 304-306 demonstrate the high angular
resolution performance of the distributed aperture automotive radar
systems and methodologies disclosed herein. With the angle spectrum
304 for a MIMO beamforming output of a single radar, the three
targets cannot be resolved. This failure arises from the fact that
the true target spatial frequencies 301-303 for the three targets
are all contained in a single broad main central lobe instead of
three distinct main lobes at the angle positions of the three
targets.
In contrast, the angle spectrum 305 for a MIMO beamforming output
of dual radars shows that the three targets can be resolved when
two radars are combined in a coherent and alternating fashion when
forming the MIMO aperture. This is seen in FIG. 4 where the angle
spectrum 305 includes individual lobes 305A-C which, respectively,
contain the true spatial frequencies 301-303 for the three targets.
However, the angle spectrum 305 also includes large spurious side
lobes 305D-F which result in a higher false target detection
rate.
In the waveform 306 based on the forward and backward difference
co-array virtual array output, the spurious side lobes are
suppressed. This is seen in FIG. 4 where the angle spectrum 306
includes individual lobes 306A-C which, respectively, contain the
true spatial frequencies 301-303 for the three targets, and also
includes suppressed spurious side lobes (e.g., 306D-E).
As will be appreciated from the foregoing, without coherently
combining the radars, distributed radar outputs can only be
combined in a non-coherent fashion which results in no impact in
the size of aperture and it only improves the SNR or sensitivity
performance of the radar. However, coherent combination of the
radars not only improves the angular resolution, but also improves
the SNR performance. In addition, it is noted that the principle of
forward/backward difference co-array virtual array processing can
be applied to any physical or virtual arrays, and is not limited to
MIMO virtual arrays.
To provide additional details for an improved understanding of
selected embodiments of the present disclosure, reference is now
made to FIG. 5 which depicts a simplified flow chart 500 showing
the logic for combining multiple distributed small-aperture radars
to form a virtually large coherent aperture. In an example
embodiment, the control logic and methodology shown in FIG. 5 may
be implemented as hardware and/or software on a host computing
system, processor, or microcontroller unit that includes processor
and memory for storing programming control code for constructing
and operating a large virtual aperture radar by coherently
combining distributed small aperture radars which do not share a
common local oscillator signal.
The process starts (step 501), such as when a new radar frame is
started. In radar systems, the transmitted radar signal may be
periodically modulated, such as by applying a frequency and/or
phase shift. The period is typically chosen such that the radar
signal modulation occurs between two time frames of the signal,
where a time frame may, for example, correspond with a "chirp" in
an FMCW (frequency modulation continuous wave) signal.
At step 502, one of the distributed radars is selected or
designated as the master unit, and any remaining distributed radar
is selected or designated as a slave unit. The result of this
selection is that, when the selected master-unit unit transmits on
any transmit antenna, the other slave unit(s) turn off their
transmit antennas and operate only in receiver mode. As the process
iteratively repeats itself through steps 504-508 as described
below, the processing at step 502 sequentially alternates the
master unit role amongst the distributed radars by selecting a new
master radar at each iteration until all of the radars have been
selected to operate as the master unit. In each iteration, the
selected master-unit radar may sequentially transmit radar
waveforms from each transmit antenna on the master unit radar, and
the target returns from master radar's transmitted waveforms are
directionally received at the receiver antenna(s) of the master
radar and also at the receiver antenna(s) of the designated
slave-unit radar(s). To initiate operations at the distributed
master and slave units, a trigger may be sent to all units to
signal the start of a chirp.
At step 504, each slave-unit radar processes the master radar's
transmitted waveforms by applying predetermined radar signal
processing steps to the radar waveforms received at each slave
unit's receiver antenna(s). While any suitable radar signal
processing steps may be used, each slave-unit radar may be
configured to perform the same fast-time and slow-time processing
on the received radar signal in the same way it processes its own
target returns range and Doppler processing steps for the radar
waveforms received at each slave unit's receiver antenna(s). For
example, the slave-unit radar may apply range and Doppler FFT
processing to generate mono-static and bi-static range Doppler
maps.
At step 506, each slave-unit radar (or the radar controller)
calculates or estimates frequency and phase offset values, and then
applies the offsets to compensate for frequency and phase
differences between the master-unit radar and slave-unit radar.
While any suitable estimation technique may be used to calculate
the chirp starting frequency offset (.DELTA.f.sub.0) and
master-slave phase offset (.DELTA..phi.), selected embodiments of
the present disclosure configure each slave-unit radar may to
estimate these values based on the differences between the
estimated range and Doppler measurements with the known truth. More
specifically, a signal processing algorithm is implemented with
software instructions which are executed to directly derive the
frequency offset (.DELTA.f.sub.0) from the Doppler position of the
identified eavesdropped signal peak on the range-Doppler map. In
addition, the master-slave phase offset (.DELTA..phi.) may be
solved from the instantaneous frequency model:
.function..times..DELTA..times..times..DELTA..times..times..times..DELTA.-
.times..times..times..times..times..DELTA..times..times..times..DELTA..tim-
es..times..times..DELTA..times..times..times..DELTA..times..times..times..-
DELTA..times..times..DELTA..phi..times..pi. ##EQU00011## based on
the range-FFT estimated value of f.sub.ms(t) and the known or
negligible values of f.sub.0, .DELTA.f.sub.0, .DELTA.t.sub.0, {dot
over (f)}, and .DELTA.{dot over (f)}. Once the frequency and phase
offset values are derived for each slave-unit radar, the slave
radars apply the estimated frequency and phase offsets to produce
coherent target measurements. In this way, each slave-unit radar
(or radar controller) processes the slave-received master signal
data to estimate the slave unit's frequency and phase offsets to
the master unit's, thereby allowing the slave-unit radar to
generate coherent target data samples that are correlated in time,
frequency, and phase with the master-unit radar.
At step 508, the correlated target return data samples received
from the distributed radar devices are processed using bi-static
radar principles to construct and accumulate mono-static and
bi-static MIMO virtual array outputs by combining the distributed
apertures, but without requiring physically mixing the received
master-unit's transmissions with received target returns or sharing
a local oscillator signal. In selected embodiments, the processing
of correlated target return data samples generated by each
master-unit is performed at the radar controller processor to
generate a bi-static MIMO virtual array which includes a
mono-static MIMO virtual array elements (e.g., 204) and bi-static
MIMO virtual array elements (e.g., 205).
At step 510, the process determines if all of the distributed
radars have been designated to operate as the master-unit radar. If
not (negative outcome to detection step 510), then the process
returns to step 502 to select another one of the distributed radars
as the new master-unit, and step 504-510 are repeated until all
radars have been selected as the master unit once. However, if the
last radar has been selected as a master-unit (affirmative outcome
to detection step 510), then the process may proceed to perform
additional processing.
At step 512, the accumulated MIMO virtual array outputs generated
at step 508 are processed to generate a first set of beamforming
outputs that define an extended MIMO virtual aperture based on MIMO
radar principles. In selected embodiments, the processing of MIMO
virtual array outputs is performed at the radar controller
processor to generate a first set of beamforming outputs using a
selected least-sparse section of the MIMO virtual array.
At step 514, the MIMO array outputs are used to construct forward
different co-array outputs, alone or in combination with backward
difference co-array outputs. In selected embodiments, the forward
difference co-array outputs are constructed by the radar controller
processor which determines relative distance positions of the
antenna elements in the MIMO array, identifies all combinations of
antenna element pairs x.sub.i, x.sub.j=x.sub.i-x.sub.j having a
zero or positive difference spacing, and then calculates virtual
forward co-array element outputs. If desired, angle processing,
such as beamforming, can then be carried out based on the forward
difference co-array construction, resulting in a bi-static MIMO
forward difference virtual co-array aperture (e.g., 200D) that is
the same size as, but less sparse than, the bi-static MIMO virtual
array aperture (e.g., 200C). In addition, the radar controller
processor may construct the backward difference co-array outputs by
identifying all combinations of antenna element pairs x.sub.i,
x.sub.j=x.sub.i-x.sub.j having a zero or negative difference
spacing and then calculating virtual backward co-array element
outputs. If desired, angle processing can then be carried out based
on the forward and backward difference co-array construction,
resulting in a bi-static MIMO forward/backward difference virtual
co-array aperture (e.g., 200E) that is larger than, and less sparse
than, the bi-static MIMO virtual array aperture (e.g., 200C). In
selected embodiments, the bi-static MIMO forward/backward
difference virtual co-array aperture is almost twice as large as
the bi-static MIMO virtual array aperture.
At step 516, additional processing is applied to suppress or reduce
spurious side lobes by performing spatial smoothing on the
forward/backward difference co-array outputs if the MIMO array is
uniformly spaced. In selected embodiments, the radar controller
processor may be configured to spatially smooth the
forward/backward difference co-array element outputs in the forward
direction. However, if the formed virtual array does not have
equally spaced antenna elements, then step 516 is skipped.
At step 518, the (spatially smoothed) forward/backward difference
co-array outputs are processed to generate a second set of
beamforming outputs that define an extended MIMO virtual aperture
based on MIMO radar principles. In selected embodiments, the
processing of the forward/backward difference co-array outputs is
performed at the radar controller processor. The size of the
resulting aperture is almost doubled and the aperture is fuller,
which results in improved angular resolution and improved spurious
sidelobe performance.
At step 520, additional processing is applied to suppress or reduce
spurious side lobes by computing composite beamforming outputs. In
selected embodiments, the radar controller processor may be
configured to produce a composite beam forming output by
multiplying the beam forming output of the forward/backward
difference co-array (with or without spatial smoothing applied)
with the beam forming output of a section of the MIMO virtual
array. By doing so, the spurious sidelobes in the output of the
difference co-array process are greatly suppressed.
As will be appreciated by persons skilled in the art, the computed
difference co-array output can be further processed using any
super-resolution angle estimation algorithms, included but not
limited to the beamforming algorithms based on Fourier analysis of
the spatial frequency components of the co-array outputs, such as
Discrete Fourier Transform or Fast Fourier Transform. Alternative
super-resolution angle estimation algorithms include, but not limit
to, Multiple Signal Classification (MUSIC) algorithm and its
derivatives, Rotational Invariance (ESPRIT) algorithm and its
derivatives, Matrix Pencil algorithm and its derivatives, Method of
Direction Estimation (MODE) algorithm, Noise or Signal Subspace
Fitting algorithm or its derivatives, Maximum Likelihood Estimator
based algorithms, and Sparsity Constraint based or L1-Norm
minimization based algorithms, among others.
In addition, it will be understood that the forward and backward
difference co-array processing can be applied to a distributed
radar system whose apertures are physically separated but share a
common LO signal via a physical link. In this case, the
eavesdropping processing steps are removed. In addition, the
forward and backward difference co-array processing can be applied
to a single radar system whose either physical antenna array or
virtual MIMO array is sparse. In this case, the processing steps
reduce to mono-static case and the need for making two distributed
radar coherent is removed, as there is only a single coherent
radar.
At step 522, the target map is generated to identify the range,
Doppler, and angle values for each detected target. In selected
embodiments, the radar controller processor may be configured to
produce map data identifying paired range (r), Doppler ({dot over
(r)}) and angle (.theta.) values for each detected/target
object.
As disclosed herein, selected embodiments of the disclosed
distributed aperture radar system may provide several enhancements
when compared with conventional radar systems. In addition to
enabling the construction of a single large coherent aperture from
two or more distributed radars which achieves high angular
resolution and suppresses spurious side lobes, the disclosed
distributed aperture radar system can use RF front-end and signal
processing blocks of existing radar designs without modifications,
thereby minimizing the cost of developing the new solution. In
addition, the distributed aperture radar system disclosed herein
eliminates the requirement of physically sharing the LO signals at
each radar device by providing an efficient mechanism for using
estimated frequency and phase offset values to correlate the time,
frequency and phase of distributed radar devices so that bi-static
radar principles can be used to form a coherent aperture across a
distance via eavesdropping the other radar's transmission. In
addition, the present disclosure eliminates the need to perform
cross-correlation or matched filtering since only specified mixing
and fast-time and slow-time processing steps are required for
estimating the frequency and phase offsets between radars. For
systems that operate on independent, low cost oscillators, the
ability to efficiently derive the frequency offset values is most
helpful. In addition, when distributed radars are separated at a
distance, the formed apertures are typically sparse in nature,
resulting in spurious sidelobes, so the ability to mitigate or
suppress the side lobes as disclosed herein enables larger
separation between the radars without significantly increasing
false detections.
By now it should be appreciated that there has been provided a
distributed radar architecture, circuit, method, and system for
coherently combining physically distributed radars by applying fast
and slow time processing steps at a receiving radar to derive
estimated frequency and phase offsets between the distributed
radars for use in coherently combining the radars without sharing a
common local oscillator reference. In the disclosed embodiments,
the distributed aperture radar system includes first and second
small aperture radar devices that are physically distributed from
one another and connected to a radar control processing unit. The
first small aperture radar device includes a first plurality of
transmit and receive antennas, a first local oscillator reference
clock generator, and a first signal processor component coupled to
the first plurality of transmit and receive antennas to transmit a
first radar signal and to generate a mono-static virtual array
aperture by processing target returns received at one or more
receive antennas of the first small aperture radar device in
response to the first radar signal. In addition, the second small
aperture radar device includes a second plurality of transmit and
receive antennas, a second independent local oscillator reference
clock generator, and a second signal processor component coupled to
the second plurality of transmit and receive antennas. As
connected, the second signal processor component is configured to
process target returns and an eavesdropped signal of the first
small aperture radar device received at one or more receive
antennas of the second small aperture radar device in response to
the first radar signal with fast and slow time processing steps.
For example, the fast and slow time processing steps may be
performed by applying range and doppler processing steps to the
target returns to generate a range-doppler map. In selected
embodiments, the second signal processor component performs range
and doppler processing steps by computing range fast Fourier
transforms (FFTs) and doppler FFTs on target return signals
received at the one or more receive antennas of the second small
aperture radar device to generate the range-doppler map. The second
signal processor component is also configured to compute an
estimated frequency offset and an estimated phase offset between
the first and second small aperture radar devices based on
information derived from the fast and slow time processing steps.
In selected embodiments, the second small aperture radar device
computes the estimated frequency offset from the range-doppler map
based on doppler and range positions of an identified eavesdropped
signal peak in the range-doppler map. In other embodiments, the
second small aperture radar device computes the estimated phase
offset from the range-doppler map based on a detected phase of a
peak amplitude of an identified eavesdropped signal peak in the
range-doppler map. In addition, the second signal processor
component is configured to apply the estimated frequency offset and
estimated phase offset to generate a bi-static virtual array
aperture that is coherent in frequency and phase with the
mono-static virtual array aperture. The radar control processing
unit is coupled to the first and second small aperture radar
devices and configured to produce target scene information by
coherently combining the bi-static virtual array aperture and the
mono-static virtual array aperture to construct an extended
bi-static virtual array aperture that is larger than either the
bi-static virtual array aperture or the mono-static virtual array
aperture. In selected embodiments, the first and second small
aperture radar devices may each be formed as a system-on-a-chip
(SOC). In other embodiments, the extended bi-static virtual array
aperture may be a bi-static multiple-input multiple-output (MIMO)
virtual array aperture.
In another form, there is provided a method, system, and apparatus
for operating a distributed aperture radar system which includes
first and second small aperture radar devices that are physically
distributed from one another and connected to a radar control
processing unit. In selected embodiments, the first small aperture
radar device includes a first plurality of transmit and receive
antennas, a first local oscillator reference clock generator, and a
first signal processor component coupled to the first plurality of
transmit and receive antennas to transmit the first radar signal
and to generate the mono-static virtual array aperture. In
addition, the second small aperture radar device may include a
second plurality of transmit and receive antennas, a second
independent local oscillator reference clock generator, and a
second signal processor component coupled to the second plurality
of transmit and receive antennas to generate the bi-static virtual
array aperture. In the disclosed methodology, a first small
aperture radar device transmits a first radar signal from a first
transit antenna and then processes target returns received at one
or more receive antennas of the first small aperture radar device
in response to the first radar signal to generate a mono-static
virtual array aperture. The second small aperture radar device
processes target returns and an eavesdropped signal of the first
small aperture radar device received at one or more receive
antennas of the second small aperture radar device in response to
the first radar signal with fast and slow time processing steps. In
addition, the second small aperture radar device computes an
estimated frequency offset and an estimated phase offset between
the first and second small aperture radar devices based on
information derived from the fast and slow time processing steps.
In selected embodiments, the second small aperture radar device
computes the estimated frequency offset and estimated phase offset
by applying range and doppler processing steps to the target
returns to generate a range-doppler map. In such embodiments, the
second small aperture radar device may compute the estimated
frequency offset from the range-doppler map based on doppler and
range positions of an identified eavesdropped signal peak in the
range-doppler map. In addition or in the alternative, the second
small aperture radar device may compute range fast Fourier
transforms (FFTs) and doppler FFTs on target return signals
received at the one or more receive antennas of the second small
aperture radar device to generate the range-doppler map. At the
second small aperture radar device, the estimated frequency offset
and estimated phase offset are applied to generate a bi-static
virtual array aperture that is coherent in frequency and phase with
the mono-static virtual array aperture. As a result, target scene
information may be produced by coherently combining the bi-static
virtual array aperture and the mono-static virtual array aperture
to construct an extended bi-static virtual array aperture that is
larger than either the bi-static virtual array aperture or the
mono-static virtual array aperture. In selected embodiments, the
extended bi-static virtual array aperture may be constructed as a
bi-static multiple-input multiple-output (MIMO) virtual array
aperture.
In yet another form, there is provided a distributed aperture
bi-static radar system, method, and apparatus which includes a
first and second small multi-antenna radar devices that are
physically distributed from one another and that have independent
reference clock generators. In particular, the first multi-antenna
radar device includes a first local oscillator reference clock
generator and a first signal processor component coupled to a first
plurality of transmit and receive antennas to transmit a first
radar signal and to generate a mono-static virtual array aperture
by processing target returns received at one or more receive
antennas of the first small aperture radar device in response to
the first radar signal. In addition, the second multi-antenna radar
device includes a second, independent local oscillator reference
clock generator and a second signal processor component coupled to
a second plurality of transmit and receive antennas to process
target returns and an eavesdropped signal from the first
multi-antenna radar device received at one or more receive antennas
of the second multi-antenna radar device in response to the first
radar signal to generate a bi-static virtual array aperture that is
coherent in frequency and phase with the mono-static virtual array
aperture determining a frequency offset value .DELTA.f.sub.0, a
phase offset value .DELTA..phi., and a timing offset value
.DELTA.t.sub.0 between the first and second multi-antenna radar
devices. In selected embodiments, the second signal processor
component is configured to (1) process target returns and an
eavesdropped signal of the first multi-antenna radar device
received at one or more receive antennas of the second
multi-antenna radar device in response to the first radar signal
with fast and slow time processing steps, (2) compute an estimated
frequency offset and an estimated phase offset between the first
and second small multi-antenna radar devices based on information
derived from the fast and slow time processing steps, and (3) apply
the estimated frequency offset and estimated phase offset to
generate a bi-static virtual array aperture that is coherent in
frequency and phase with the mono-static virtual array aperture. In
such embodiments, the second multi-antenna radar device performs
fast and slow time processing steps by applying range and doppler
processing steps to the target returns to generate a range-doppler
map. For example, the second multi-antenna radar device may compute
the estimated frequency offset from the range-doppler map based on
doppler and range positions of an identified eavesdropped signal
peak in the range-doppler map. In addition or in the alternative,
the second multi-antenna radar device may compute the estimated
phase offset from the range-doppler map based on a detected phase
of a peak amplitude of an identified eavesdropped signal peak in
the range-doppler map. In addition or in the alternative, the
second multi-antenna radar device may compute range fast Fourier
transforms (FFTs) and doppler FFTs on target return signals
received at the one or more receive antennas of the second
multi-antenna radar device to generate the range-doppler map. The
disclosed distributed aperture bi-static radar system may also
include a radar control processing unit coupled to the first and
second multi-antenna radar devices and configured to produce target
scene information by coherently combining the bi-static virtual
array aperture and the mono-static virtual array aperture to
construct an extended bi-static virtual array aperture that is
larger than either the bi-static virtual array aperture or the
mono-static virtual array aperture.
Although the described exemplary embodiments disclosed herein focus
on example automotive radar circuits, systems, and methods for
using same, the present invention is not necessarily limited to the
example embodiments illustrate herein. For example, various
embodiments of a distributed aperture radar may be applied in
non-automotive applications, and may use additional or fewer
circuit components than those specifically set forth. Thus, the
particular embodiments disclosed above are illustrative only and
should not be taken as limitations upon the present invention, as
the invention may be modified and practiced in different but
equivalent manners apparent to those skilled in the art having the
benefit of the teachings herein. Accordingly, the foregoing
description is not intended to limit the invention to the
particular form set forth, but on the contrary, is intended to
cover such alternatives, modifications and equivalents as may be
included within the spirit and scope of the invention as defined by
the appended claims so that those skilled in the art should
understand that they can make various changes, substitutions and
alterations without departing from the spirit and scope of the
invention in its broadest form.
Benefits, other advantages, and solutions to problems have been
described above with regard to specific embodiments. However, the
benefits, advantages, solutions to problems, and any element(s)
that may cause any benefit, advantage, or solution to occur or
become more pronounced are not to be construed as a critical,
required, or essential feature or element of any or all the claims.
As used herein, the terms "comprises," "comprising," or any other
variation thereof, are intended to cover a non-exclusive inclusion,
such that a process, method, article, or apparatus that comprises a
list of elements does not include only those elements but may
include other elements not expressly listed or inherent to such
process, method, article, or apparatus.
* * * * *
References